ABSTRACT THE water management simulation model, DRAIN-MOD, was modified to quantify stresses on corn caused by both excessive and deficient soil water condi-tions and to weigh those stresses according to the stage of the growing season and crop susceptibility at the time they occur. Methods were added to predict the planting date and thus determine any delays in planting that might occur due to the drainage system design. An ap-proximate crop response model was used to predict an-nual crop yields in terms of stress-day indexes for defi-cient and excessive soil water conditions and the delay in planting date. The methods are demonstrated in the paper by simulating the performance of several subsurface and surface drainage designs for a Tomotly sandy loam soil near Wilmington, NC. The results showed that, in the absence of irrigation, the long-term average yield will be maximized by a subsurface drain spacing of 20 m or less for good surface drainage. For poor surface drainage a subsurface drain spacing of 12 m would be required to obtain the same yield. The effect of surface drainage on yields is particularly important when subsurface drainage is poor. For example, good surface drainage, on a field with a 90 m subsurface drain spacing, will more than double the long-term average corn yield over that obtained for poor surface drainage on the same field. Irrigation can be used to increase average corn yields by 28 percent on the soil considered if good drainage is provided. However, better drainage is required to pro-duce maximum yields when irrigation is used than when it is not. Potential yield increases due to irrigation will not be realized if adequate drainage is not provided.